The invention relates generally to slug calorimetry. In particular, the invention relates to directional slug calorimeters that are useful for heat flux measurements in extreme environments.
In order to adequately characterize a thermal environment, both temperature and heat flux must be measured. Temperature is a thermodynamic property of a material and can be easily measured by standard methods. Heat flux, on the other hand, is an energy flow rate per unit area and is therefore far more difficult to obtain. There are two methods commonly utilized for heat flux measurement. The first method measures the temperature drop across a known thermal resistance. This temperature drop can then be related to heat flux by using Fourier's law of heat conduction. Sensors of this type are called differential heat flux sensors.
The second type of heat flux sensor examines the time-rate of change of the temperature of a material that is exposed to a heat flux and isolated from all other thermal influences. When heat flows into an object, its temperature increases. The rate of temperature rise is proportional to the applied heat flux and inversely proportional to the thermal mass of the object. By measuring this rate of temperature rise and calculating the thermal mass of the material, the heat flux into the device can be determined. These devices are typically referred to as slug calorimeters heat flux sensors.
While both types of sensors have been developed and are commercially available, current commercially available sensors typically cannot survive in the extreme environments found in fire research. The high temperatures and high heat fluxes destroy these gages in a short time, rendering them useless in many situations. Even sensors made entirely from high temperature materials typically fail due to stresses that develop due to mismatches in thermal expansion coefficients. These sensors frequently tear apart when heated. As can be seen, there is a need for a device that is capable of surviving extreme environments while obtaining accurate heat flux data.